1. Field of the Invention
The present invention relates to a high-frequency circuit including a stripline, and more particularly to a stripline structure with an enhanced shielding capability for suppressing ambient electromagnetic noise and electromagnetic interference such as crosstalk, and a magnetic field detector incorporating such a stripline structure.
2. Description of the Related Art
High-frequency circuits tend to cause characteristic degradation because of electromagnetic wave interference between a plurality of circuit components and transmission lines that interconnect the circuit components. Therefore, various efforts have been made to provide electromagnetic shields in high-frequency circuits that are liable to radiate unwanted electromagnetic waves or suffer external electromagnetic waves by using metal casings and metal patterns on printed-circuit boards.
In order to increase the shieldability of transmission paths, microstrip lines having metal strips exposed on the surface layer may be changed to triplate striplines which have a metal strip as an inner layer sandwiched between upper and lower ground layers. The triplate striplines are less susceptible to at least direct electromagnetic radiations from mounted circuit components and external electromagnetic waves, and contribute to minimizing overall circuit characteristic deterioration.
However, even triplate striplines fail to provide a complete shielding capability as they suffer crosstalk between adjacent striplines in the same layer. According to one solution, as shown in FIG. 1 of the accompanying drawings, upper and lower ground layers 2002 are electrically connected to each other by vias 2003 on opposite sides of an inner conductor 2001 in surrounding relationship to the inner conductor 2001. These vias are provided in a plurality in the direction of the transmission path for increased isolation from an adjacent stripline.
Although triplate striplines are less advantageous than microstrip lines in terms of the mounting of circuit components, it is effective to apply highly shielded triplate striplines if spaced circuit components reed to be interconnected over a relatively long distance. Consequently, it is advisable to use microstrip lines in the vicinity of mounted circuits and use triplate striplines to interconnect circuit components over a relatively long distance.
For connecting a high-frequency circuit module which comprises a multilayer circuit board to another spaced high-frequency circuit module which comprises a multilayer circuit board, it is often customary to use highly shielded transmission paths such as coaxial lines between transmission and reception terminals of those high-frequency circuit modules.
FIG. 2 of the accompanying drawings shows a simplest way of connecting a coaxial line to a stripline on a multilayer circuit board.
As shown in FIG. 2, a triplate stripline 2103 has an inner conductor 2101 sandwiched between two ground patterns 2102 in a multilayer circuit board 2100. The inner conductor 2101 is electrically connected through a via 2104 to a pad 2106 of certain area on an uppermost layer 2105. The pad 2106 is electrically joined to a central conductor 2108 of a coaxial line 2107 by a solder body 2109 or the like. The ground layers 2102 are electrically connected through vias 2110 to a pad 2111 on the uppermost layer 2105, which is electrically joined to an outer conductor 2112 of the coaxial line 2107 by a solder body 2113 or the like.
An application for patent has been filed for a shielded-loop magnetic field detector using such a triplate stripline (see Japanese patent application No. 10-346030). FIG. 3 of the accompanying drawings shows the disclosed shielded-loop magnetic field detector using a triplate stripline.
As shown in FIG. 3, an inner conductor 2202 is disposed on an unexposed layer of a multilayer circuit board 2201 which comprises at least three layers, and upper and lower ground patterns 2203 are disposed above and below the inner conductor 2202 with insulators interposed therebetween, thus providing a triplate stripline 2204. An insulator may be disposed on the upper surface of the upper ground pattern, or the lower surface of the lower ground pattern, or both for the purpose of increasing the mechanical strength of the multilayer circuit board 2201.
On each of the ground patterns 2203, the stripline 2204 includes a lead 2205 extending as a straight portion from the left end and a loop 2206 bent at a right angle into a square shape on a right-hand side of the lead 2205. The loop 2206 has a terminal end 2207 short-circuited to the ground pattern 2203 of the stripline 2204. The loop 2206 has a gap 2208 defined in the right end thereof, and includes a square loop opening 2209 defined therein.
The inner conductor 2202 has a portion positioned at first half portions 2210 of the loops 2206, extending across the gaps 2208, and electrically connected through a via 2211 to the ground patterns 2203 at second half portions 2212 of the loops 2206.
With the above structure, an output produced by a magnetic field that crosses the loop openings 2209 is propagated as a stripline mode to the left ends of the leads 2205.
Since the inner conductor of the triplate stripline is sandwiched between the upper and lower ground layers, as described above, the inner conductor is highly shielded vertically by the ground layers, but not horizontally because it is open horizontally. Even if the triplate stripline is used to interconnect circuit components, therefore, it fails to provide a complete electromagnetic shield. It has heretofore been considered to be effective to employ wide upper and lower ground layers in order to increase the shielded capability of the inner conductor.
As shown in FIG. 1, the upper and lower ground patterns are electrically connected to each other by the vias on opposite sides of the inner conductor in surrounding relationship to the inner conductor to increase isolation from an adjacent stripline for thereby solving, to a certain extent, the problem of crosstalk between adjacent striplines in the same layer. However, if the electromagnetic field is disturbed in the vicinity of the vias, then electromagnetic interference occurs between the adjacent striplines, and hence the isolation from the adjacent stripline is not necessarily sufficient.
As shown in FIG. 2, if the stripline and the coaxial line are connected to each other, then since the central conductor 2108 of the coaxial line is exposed, it is likely to be affected by the electromagnetic field generated by nearby circuit components and interconnections. Furthermore, if the board and the coaxial line are stressed by an external action, then the sold bodies and metal patterns tend to be peeled off, resulting in poor electric connections.
If a printed-circuit board is positioned on the right-hand side of the conventional shielded-loop magnetic field detector using a triplate stripline as shown in FIG. 3 and a magnetic field near the PCB interconnections is to be detected by the magnetic field detector, then since the electromagnetic field near the PCB where a number of circuit components are mounted and connected by highly dense interconnections is not uniform, but highly disturbed, it is difficult for the shielded-loop magnetic field detector that is positioned in the disturbed electromagnetic field to detect only the desired magnetic field.
Furthermore, as described above, the inner conductor sandwiched between the upper and lower ground patterns is highly shielded vertically by the ground layers, but not horizontally because it is open horizontally. If there is a disturbed electromagnetic field near a printed-circuit board, the magnetic field detector tends to detect the unwanted electromagnetic field other than the desired magnetic field, and hence suffers poor measurement accuracy.
It is therefore an object of the present invention to provide a high-frequency circuit including a stripline with an increased shielding capability for suppressing the effect of ambient electromagnetic noise and electromagnetic interference such as crosstalk.
Another object of the present invention is to provide a shielded-loop magnetic field detector using a triplate stripline with an increased shielding capability.
According to an aspect of the present invention as described in claims 1 and 2, a high-frequency circuit includes a triplate stripline on a multilayer circuit board, the triplate stripline comprising an inner conductor sandwiched between two ground patterns having a finite pattern width greater than a pattern width of the inner conductor. The ground patterns are short-circuited to each other on opposite transverse ends thereof by a plurality of vias disposed in a longitudinal direction which is a signal transmitting direction of the stripline. In a cross section including the vias, the inner conductor is surrounded by the two ground patterns and the vias on the opposite sides thereof.
As a result, a ground current in the triplate stripline and a ground current in an adjacent stripline do not interfere with each other. Any electromagnetic interference with the high-frequency circuit is reduced, and the shielding capability of the high-frequency circuit is increased. The triplate stripline is prevented from suffering resonance which would otherwise occur in the entire multilayer circuit board due to solid plane power supply and ground patterns operating as resonators frequently on conventional multilayer circuit boards.
The high-frequency circuit as described in claims 3 and 4 may further include a coaxial line disposed on an uppermost layer of the multilayer circuit board. The coaxial line may comprise a central conductor exposed front a tip end thereof, inserted into a through hole of a via which is defined in the uppermost layer of the multilayer circuit board and electrically connected to the inner conductor of the stripline, and electrically connected by a solder body, and an outer conductor exposed near the tip end thereof and electrically connected by a solder to a metal pattern which is electrically connected to the ground patterns of the stripline. With this arrangement, connection failures between the stripline and the coaxial line can be reduced. Since the central conductor of the coaxial line is nearly directly connected to the inner conductor of the stripline, there is achieved a connection that is of excellent high frequency characteristics and provides high matching over a wide frequency range.
The high-frequency circuit as described in claims 5 and 6 may further include a metal casing which shields the ground pattern and the exposed central conductor on the uppermost layer of the multilayer circuit board. The metal casing may be held in close contact with and electrically connected to one of the ground layers of the stripline and the outer conductor of the coaxial line by a solder body, and fastened to the multilayer circuit board from opposite sides thereof by a bolt and a nut. Because the exposed central conductor of the coaxial line is surrounded by the conductor or metal casing, the central conductor is highly shielded. The metal casing provides increased mechanical strength for keeping the multilayer circuit board and the coaxial line stably connected against stresses applied thereto.
According to another aspect of the present invention as described in claims 7 to 14, there is provided a shielded-loop magnetic field detector comprising a triplate stripline which includes a lead and a loop. The triplate stripline is of the structure described above. When an output produced by a magnetic field detected by the loop is propagated as a stripline mode to the lead, because the shielding capability of the stripline is increased, a leakage of the propagated mode and the effect of ambient noise are reduced.
Even if the loop as a magnetic field detector is positioned closely to an object which exhibits a complex electromagnetic field distribution, since the loop is highly shielded , the effect of an unwanted electromagnetic field other than a desired magnetic field to be measured is reduced, and the shielded-loop magnetic field detector can measure magnetic fields highly reliably.
According to the present invention as described in claims 15 to 18, the two ground patterns at the second half portion which is free of the inner conductor may be short-circuited to each other at a transverse center thereof by a plurality of vias arranged along the second half portion.
When an output produced by a magnetic field detected by the loop is propagated as a stripline mode to the lead, because the shielding capability of the stripline is increased, a leakage of the propagated mode and the effect of ambient noise are reduced. Even if the loop as a magnetic field detector is positioned closely to an object which exhibits a complex electromagnetic field distribution, since the loop is highly shielded, the effect of an unwanted electromagnetic field other than a desired magnetic field to be measured is reduced, and the shielded-loop magnetic field detector can measure magnetic fields highly reliably.
Furthermore, since the two ground patterns at the second half portion of the loop which is free of the inner conductor are short-circuited at the opposite transverse ends and the transverse center through the vias, the impedance of the ground patterns is reduced for stable magnetic field measurement. The magnetic field detector is free of characteristics deterioration which would otherwise be caused by an unwanted mode produced by an unwanted electromagnetic field entering between the two ground patterns from the object being measured.